Photoionization gas detection is commonly used in applications where dangerous gases may infiltrate the breathing environment. A serious drawback to the implementation of photoionization gas detection in a portable instrument is the large degree of negative cross sensitivity exhibited by such sensors to water vapor. For example, the studies of Chilton, et al., American Industrial Hygiene Association Journal, 44:710 through 715 (1983), Barski, et al., American Industrial Hygiene Association Journal, 46:9-14 (1985) confirm that the response of a photoionization detector is markedly reduced with increased water vapor concentration.
In typical operation, a portable instrument using a photoionizing detector is calibrated prior to use with a span gas containing a known concentration of an ionizeable species, and a fixed, normally near zero partial pressure of water vapor. If such an instrument is subsequently used to sample an ambient atmosphere containing the same contaminant at the same concentration level, the instrument will invariably register a value lower than the calibration reading due to the presence of water vapor in the field sample. As the ambient atmosphere always contains some degree of humidification, in actual operation instruments of this type are susceptible to gross underestimations of the toxicity level of the surrounding breathing environment.